The present invention relates to a method for producing millable, polyester polyurethanes having increased resistance to hydrolysis while maintaining strength inherent in polyurethanes.
Polyurethane elastomers are classified into castable polyurethanes, millable polyurethanes, thermoplastic polyurethanes. Among these, castable polyurethanes are generally used because they have an excellent combination of properties. On the other hand, the millable polyurethanes are advantageous in that they can be processed using rolls, presses and like devices generally employed for processing rubbers.
The castable and thermoplastic polyurethanes are known to be representative examples of segmented elastomers such as thermoplastic elastomers and have both rubber elasticity and wear resistance and strength. When properties such as tear strength and wear resistance are important, polyurethanes have been used which comprise soft segments comprising a crystallizable long polyol chain such as polyethylene glycol adipate, polybutylene glycol adipate, polyhexanediol adipate, poly-.epsilon.-caprolactone and hard segments comprising a polyisocyanate and a short chain polyol and polyamine, with the hard segments preventing the crystallization of the soft segments. More specifically, under usual conditions, the ends of the soft segments are fixed randomly by the hard segments so that the crystallization of the soft segments is prevented to form amorphous polymer chains. As a result, the polymer as a whole exhibits rubber elasticity and when the polymer is deformed excessively, the soft segments, fixed randomly, exhibit orientation crystallizability. Accordingly, polyurethanes vary in the crystal conditions of the soft segments when they are under usual conditions or when they are under excessive deformation. This is why polyurethanes are excellent in wear resistance and strength. Thus, polyurethanes of the castable and thermoplastic types can contain high crystallizability polyols.
On the other hand, in the case of millable polyurethanes, which do not have the hard segments that are present in the castable or thermoplastic type polyurethanes, use of high crystallizability polyols causes crystallization at low temperatures or at normal temperatures, thus failing to show elastomeric behavior. As described above, in designing of the millable polyurethanes, it is impossible to use those polyols that show high crystallizability at ordinary conditions and, hence, various efforts have been made to design polyols so that they have a decreased crystallizability, such as introduction of side chains, random copolymerization, and the like. Under the circumstances, the polyols put in practice include polyester-based ones such as polyethylene propylene adipate and polyethylene butylene adipate, polyether-based ones such as tetrahydrofuran (THF)-alkyl glycidyl ether copolymers, and the like, which are polyols prepared by random polymerization so that structural regularity of the polymer is disturbed to decrease its crystallizability.
Further, the millable polyurethanes are classified into polyester polyurethanes and polyether polyurethanes. Generally, the polyester polyurethanes have low resistance to hydrolysis while the polyether polyurethanes are poor in resistance to thermal aging. The poor resistance to thermal aging of the polyether polyurethanes is an essential problem while the low resistance to hydrolysis of the polyester polyurethanes is not so serious since the resistance to hydrolysis can be improved relatively readily by decreasing the concentration of the ester groups. However, in the case of polyester polyurethanes, use of long straight chain diol adipates such as 1,4-butylene adipate and 1,6-hexylene adipate in order to decrease the concentration of the ester groups results in higher crystallizability so that the resulting polyurethanes cannot be used as millable polyurethanes, particularly at low temperatures, thus failing to give a practically useful polyurethane having a wide range of feasibility. Although poly-.epsilon.-caprolactone is generally used in castable or thermoplastic type polyurethanes as a raw material for producing hydrolysis resistant polyurethanes, it cannot be used in millable polyurethanes because of its high crystallizability. Further, it is generally adopted practice to homo- or copolymerize a long chain diol having a side chain such as a methyl group in order to decrease the concentration of the ester groups and, hence, the crystallizability thereof. For example, use of a homopolymer of 3-methyl-1,5-pentanediol adipate or use of copolyester of hexanediol and neopentyl glycol, no problem of crystallization occurs at low temperatures so that elastomers having excellent hydrolysis resistance can be obtained. However, on the other hand, sufficient mechanical strength inherent to polyurethanes, such as wear resistance, cannot be obtained. When dibasic acids other than adipic acid are used, the same results are obtained. As described above, the conventional approach for solving the problems of crystallization at low temperatures has been to resort to randomly disturbing the structural regularity of polyols and therefore when the polymer is deformed excessively the crystallization is prevented so that orientation crystallization tends to occur with difficulty, thus failing to give sufficient mechanical strengths that polyurethanes have inherently.